1. Field of the Invention
The invention relates to a nonhygroscopic thermally stable aluminum hydroxide and its application in thermally stable fire retardant laminates for printed circuit boards.
2. Background Art
Printed circuit board laminate types are defined by the American NEMA (National Electrical Manufacturers Association) standard; the terminology is accepted world-wide. Generally, laminates are categorized according to the type of reinforcement used, i.e., cellulose paper or glass. Typically, the type FR-2 uses paper only, and CEM-1 uses paper and woven glass, while type CEM-3 contains both woven and non-woven glass. The type FR-4 contains woven glass only.
To achieve the required V0 level of fire retardancy according to American Underwriters Laboratories' standard UL-94, it is necessary to add either fire retardant chemicals to the polymer system or build halogens or phosphorous into the backbone of the polymer itself. On combustion, these additives help to extinguish the fire. However, in the process of doing so, they produce toxic and corrosive gases. In the case of the combustion of FR-2 laminates, which contain phosphorous compounds, phosphoric acid is formed. CEM-1, CEM-3 and FR4 laminates, which contain brominated epoxy resin, produce corrosive and toxic hydrogen bromide on combustion.
It is well known in the art that aluminum hydroxide may be used to improve the fire retardancy of synthetic polymer systems based on, for example, epoxy, polyester, and vinyl ester, because the polymers decompose in the same temperature range as the aluminum hydroxide. However, the thermal stability of the gibbsite form of aluminum hydroxide [Al(OH).sub.3 or sometimes represented as Al.sub.2 O.sub.3.3H.sub.2 O] is insufficient at the temperatures used to solder components to a printed circuit board laminate. This can result in blistering of the laminate, thereby rendering it unusable.
It is known that when gibbsite type aluminum hydroxide is heat treated in air, it is partially converted to the monohydrate form, boehmite (AlOOH or Al.sub.2 O.sub.3.H.sub.2 O), which improves thermal stability but to the detriment of fire retardancy.
Japanese Published Patent Application No. A 60/203438 describes a CEM-3 laminate containing heat treated gibbsite aluminum hydroxide which has improved thermal stability over standard aluminum trihydroxide, but which does not give the required level of fire retardancy so that brominated epoxy resin has to be used to achieve the UL 94 V0 rating. In such circumstances, and in the absence of superior fire properties, other inorganic materials, such as, talc or clay, could also be used.
British Published Patent Application No. 9700708.2 describes superior CEM-3 laminates having good thermal stability, being halogen/phosphorus free and meeting the UL 94 V0 requirement by introducing a thermally stable aluminum hydroxide to the laminate.
However, it has been found that these thermally stable aluminum hydroxides have a tendency to absorb moisture. This moisture significantly reduces solder resistance of the laminates. The thermally stable aluminum hydroxide needs accordingly and optionally to be applied more or less immediately after production. It has been found that the moisture may become firmly bound on the surface with the effect that the techniques commonly applied to remove surface moisture, such as, prior heat treatment at 110.degree. C. or azeotropic removal with the acetone used in the laminating system, may not satisfactorily work.